# Terminology: Omega_0 = Omega_m? and more

1. Jul 7, 2008

### hAndrew

Some papers refer to Omega_0 without defining it. (Example: The first footnote on page 2 in http://arxiv.org/PS_cache/astro-ph/pdf/9908/9908159v3.pdf.) What's the normal assumption in this case? Omega_0 = Omega_m (Omega_m as in Omega_m + Omega_Lambda = 1 in LambdaCDM) fits the context, but seems strange in the above paper because the authors use the notation Omega_m elsewhere in the paper. Is Omega_0 = Omega_m?

A related question: That paper refers to the "universal density" rho_u. I haven't heard that term before. rho_u = Omega_m * rho_critical fits the context. Is "universal density" = Omega_m * rho_critical?

Last edited: Jul 7, 2008
2. Jul 10, 2008

### hellfire

Usually $\Omega_0$ refers to the fraction of the total density and the critical density today $\Omega_0 = \rho_0 / \rho_{c,0}$. The term "universtal density" in that paper seems to me to be equivalent to the density at a given redshift, but I might be wrong.

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